The present invention relates generally to streak cameras and more particularly to a trigger circuit for producing a trigger pulse for triggering the sweep drive circuitry in a streak camera.
Streak cameras are about fifteen years old in the art and have been used hitherto, primarily, to directly measure the time dynamics of luminuous events, i.e., to time resolve light pulses. A typical streak camera comprises an entrance slit, input relay optics, a streak camera tube having disposed therein a photocathode, an accelerating mesh, sweeping electrodes, electron multiplication means and a phosphor screen, sweep drive circuitry (including a sweep voltage generator) for driving the sweeping electrodes, accellerating means for accellerating electrons emitted from the photocathode to the phosphor screen, and output-relay optics. The operation of a streak camera is as follows: A light pulse is projected onto the entrance slit. The slit-image of the incident pulse of light is then focused on the photocathode of the streak camera tube via the relay lens. The photocathode converts the pulse of light focused thereon into electrons which are then emitted therefrom. The electrons are then accelerated from the photocathode, through the accelerating mesh and, thereafter, into a deflection field established between the sweeping electrodes. Once in the deflection field, the electrons are swept at high speed in a direction perpendicular to the slit length. To ensure that all of the electrons will be deflected during their passage between the sweep electrodes, the arrival of the deflection voltage at the sweep electrodes is synchronized with the arrival of the emitted electrons thereat. After being deflected by the sweep electrodes, the electrons are multiplied, typically by a microchannel plate. Electrons exiting the microchannel plate then bombard the phosphor screen of the streak tube and are converted into an optical image (called a "streak image").
As a result of this sturcture and the sweeping system used, the time at which the electrons are released from the photocathode surface can be determined by measuring their degree of deflection (vertical position on the phosphor screen). Therefore, the time axis of the incident light corresponds to the vertical axis on the phosphor screen, and the intensity of the incident light can be determined by the density of the streak image.
Since it is necessary that the timing of the high speed deflection be synchronized with the arrival time of electrons at the sweep electrodes, streak cameras very often split the pulse of light into two pulses. The first pulse or luminescence emitted from a device when excited by the first pulse is focused on the photocathode in the manner described above. The pulse will ultimately go on to form the image. The second pulse is directed to a light sensitive switch which is usually in the form of a PIN photodiode detector. When the second pulse impinges upon the PIN photodiode detector, a voltage signal of intensity proportional to the second pulse is emitted therefrom. This voltage signal is used to trigger the transmission of a deflection voltage from the sweep drive circuitry to the sweep electrodes.
In U.S. Pat. No. 4,645,918 issued to Tsuchiya et al., there is disclosed an instrument, including a streak camera, for time resolving successive light pulses generated at a high repetition rate. The streak camera disclosed includes a PIN photodiode detector which is used to synchronize the arrival of the deflection voltage at the sweep electrodes with the arrival of the emitted photoelectrons. The synchronization works as follows: The pulse of light to be examined is split into first and second pulses. The first pulse is focused on the photocathode, which converts the first pulse into emitted electrons which are then accelerated towards the sweep electrodes. The second pulse is directed to a PIN photodiode. The PIN photodiode generates pulse currents in response to the second pulse of light, which impinges thereon. The output of the PIN photodiode is then amplified by an amplifier and sent to a delay circuit. After propagating through the delay circuit, whose variable path length is the determinant of the degree of delay, the signal is sent to a tuned amplifier where another voltage is generated. This voltage, which in turn is amplified by a drive amplifier, is then transmitted to the sweep electrodes for defection of the electrons passing therebetween.
In U.S. Pat. No. 4,661,694 issued to Corcoran, there is disclosed a streak camera having a plate of nonlinear non-phase-matchable material in combination with a photocathode of high sensitivity to visible radiation which renders the streak camera capable of time resolving infrared incident pulses. Synchronization of the deflection field with the arrival of the photoelectrons is similarly accomplished by means of a PIN photodiode which converts the infrared pulse being tested into a trigger signal used to activate a sweep voltage generator coupled to the sweep electrodes.
In U.S. Pat. No. 4,327,285 issued to Bradley, there is disclosed a streak camera for use in time resolving repetitive optical phenomena of picosecond or faster duration. In one embodiment of the invention, synchronization of the deflection field with the arrival of photoelectrons thereat is achieved by detecting the light pulses with a photo-detector and then feeding an electrical output signal therefrom to a sync circuit to lock the driving voltage from a signal generator to the pulse train.
One problem common to the above described streak cameras is their susceptibility to a phenomenon known as trigger jitter, which results in the projection of successive streak images produced at a high repetition rate on the phosphor screen at different loci. Because the successive streak images are often projected into the phosphor screen at different places, averaging of successive streak images is frequently made very difficult (or even impossible where the streak image is projected outside of the phosphor screen). Trigger jitter is caused by the inherent fluctuations in the amplitudes of most light pulse sources. Lasers, for instance, are known to fluctuate in amplitude by more than a factor of two or three. The reason why this fluctuation in amplitude results in trigger jitter in all of the above described streak cameras is that all of the synchronization systems make use of a PIN photodiode as a means for generating trigger pulses. Consequently, when the light pulses of varying amplitude impinge on the photodiode, trigger pulses, which are proportional in intensity to the amplitude of light impinging thereon, are emitted from the PIN photodiode and transmitted to the sweep voltage generator. Because the sweep voltage generators are all keyed to a threshold voltage and because all the trigger pulses from a PIN photodiode have a well defined profile shape and have approximately equal duration, the trigger pulses of greater intensity trigger the voltage generator sooner than trigger pulses of lesser intensity. As a result, the deflection field is established at varying times, depending on the intensity of the voltage pulse up to hundreds of picoseconds.
As noted above, a lack of synchronization between the deflection field and the arrival of photoelectrons at the sweep electrons will result in malalignment of successive streak images. To illustrate the severity of the problem posed by the trigger jitter phenomenon, it is well known in the streak camera art that fluctuations in amplitude of light pulses by more than 20% result in trigger jitter of at least 100 ps, peak to peak.
In U.S. Pat. No. 4,413,178 issued to Mourou et al., there is disclosed a sweep drive circuit for streak camera. The circuit includes a photo-conductive, solid state switch, which when activated by a pulse of light, transmits a high charging sweep voltage derived from a high DC voltage supply (output approximately 2000 volts) directly to the sweep electrodes of the streak camera. As noted, the solid state switch is a part of the sweep drive circuit itself and is not a part of a device itself for providing a trigger pulse to a sweep drive circuit.
One of the limitations of the sweep drive circuit in Mourou is that the sweeping voltage supplied to the deflection plates cannot be varied. As result, the rate at which sweeping occurs is fixed at a specific speed, dependent upon the output of the DC power supply. This is undesirable since changes in the rate of sweeping are often necessary.
It is therefore an object of the present invention to provide a new and improved circuit, suitable for use as a trigger source to the input of the sweep drive circuitry in a streak camera.
It is another object of this invention to provide a circuit as described above which eliminates trigger jitter.
It is another object of the present invention to provide a circuit as described above which does not require the use of a high voltage DC power supply.
It is still another object of the present invention to provide a circuit as described above which is inexpensive to make, which can be mass produced, which can be easily assembled, and which is easy to use.